From Ohanas, you really can only see the fireworks... still beautiful BUT you MUST request a window seat to see it when you arrive and be willing to wait... we didn't because it wasn't a big issue for us, and you could not see them from our table. You can walk around to the other side to view them but you have to bend over because of the levels in the restaurant, and that's just not very comfy for a 15 min show =0
Only those that sat near the windows seemed to view it, the people around us just kept on talking and eating. They do pipe in the music and dim the lights.
CA Grill is the other one to view it at, and i believe they have an observation deck that guests can go out and view it from... I'm not sure if you can see the castle or not.
I think there is something magical about seeing Wishes right on Main St, though it's nice to also see it from afar. I would just do it both ways =)